Male mice with large inversions or deletions of X-chromosome palindrome arms are fertile and express their associated genes during post-meiosis.

Large (>10 kb) palindromic sequences are enriched on mammalian sex chromosomes. In mice, these palindromes harbor gene families (≥2 gene copies) expressed exclusively in post-meiotic testicular germ cells, a time when most single-copy sex-linked genes are transcriptionally repressed. This observation led to the hypothesis that palindromic structures or having ≥2 gene copies enable post-meiotic gene expression. We tested these hypotheses by using CRISPR to precisely engineer large (10's of kb) inversions and deletions of X-chromosome palindrome arms for two regions that carry the mouse 4930567H17Rik and Mageb5 palindrome gene families. We found that 4930567H17Rik and Mageb5 gene expression is unaffected in mice carrying palindrome arm inversions and halved in mice carrying palindrome arm deletions. We assessed whether palindrome-associated genes were sensitive to reduced expression in mice carrying palindrome arm deletions. Male mice carrying palindrome arm deletions are fertile and show no defects in post-meiotic spermatogenesis. Together, these findings suggest palindromic structures on the sex chromosomes are not necessary for their associated genes to evade post-meiotic transcriptional repression and that these genes are not sensitive to reduced expression levels. Large sex chromosome palindromes may be important for other reasons, such as promoting gene conversion between palindrome arms.